Abstract
Deltas are resource rich, low-lying areas where vulnerability to flooding is exacerbated by natural and anthropogenically induced subsidence and geocentric sea-level rise, threatening the large populations often found in these settings. Delta ‘drowning’ is potentially offset by deposition of sediment on the delta surface, making the delivery of fluvial sediment to the delta a key balancing control in offsetting relative sea-level rise, provided that sediment can be dispersed across the subaerial delta. Here we analyse projected changes in fluvial sediment flux over the 21st century to 47 of the world’s major deltas under 12 environmental change scenarios. The 12 scenarios were constructed using four climate pathways (Representative Concentration Pathways 2.6, 4.5, 6.0 and 8.5), three socioeconomic pathways (Shared Socioeconomic Pathways 1, 2 and 3), and one reservoir construction timeline. A majority (33/47) of the investigated deltas are projected to experience reductions in sediment flux by the end of the century, when considering the average of the scenarios, with mean and maximum declines of 38% and 83%, respectively, between 1990–2019 and 2070–2099. These declines are driven by the effects of anthropogenic activities (changing land management practices and dam construction) overwhelming the effects of future climate change. The results frame the extent and magnitude of future sustainability of major global deltas. They highlight the consequences of direct (e.g. damming) and indirect (e.g. climate change) alteration of fluvial sediment flux dynamics and stress the need for further in-depth analysis for individual deltas to aid in developing appropriate management measures.
Highlights
The world’s deltas account for less than 0.5% of global land area but are home to over 5% of the global population with their fertile soils supporting intensive agriculture and important expanding cities, such that their social and economic importance extends well beyond their immediate locales (Woodroffe et al 2006, Syvitski et al 2009, Evans 2012)
Considering the ensemble mean of the 12 environmental change scenarios across all 47 deltas, mean annual sediment flux is projected to decrease by 38%
The change varies between a reduction of 83% (30 Mt/a) for the Indus to an increase of 49% (3 Mt/a) for the Limpopo
Summary
The world’s deltas account for less than 0.5% of global land area but are home to over 5% of the global population with their fertile soils supporting intensive agriculture and important expanding cities, such that their social and economic importance extends well beyond their immediate locales (Woodroffe et al 2006, Syvitski et al 2009, Evans 2012). Deltas’ low-lying land means that the inhabitants are highly exposed to the threat of rising relative sea level (Ericson et al 2006, Syvitski 2008, Ibáñez et al 2014) Associated problems, such as water and soil salinization and land loss, threaten agricultural productivity and livelihoods, presenting a challenge to future food security (Smajgl et al 2015). Deltas naturally sink relative to sea level as a result of, for instance, sediment compaction and tectonics, anthropogenic activities such as groundwater abstraction often induce accelerated subsidence (Chen et al 2012, Erban et al 2014, Brown and Nicholls 2015, Fujihara et al 2015, Higgins 2016, Jones et al 2016, Minderhoud et al 2017). Many delta areas are experiencing rates of relative sea-level rise in excess of 10 mm/a when considering geocentric sea-level rise (Chen et al 2014) in combination with land elevation change, where the latter is comprised of subsidence, crustal movement, and accretion (Syvitski et al 2009)
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